This application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2005-230863 filed in Japan on Aug. 9, 2005, the entire contents of which are hereby incorporated by reference.
The present invention relates to fixing apparatus for fixing a development image onto a sheet of recording paper in electrophotographic image forming apparatuses such as copying machines, facsimile machines, and printers.
In this type of fixing apparatus, a sheet of recording paper is sandwiched between a heating roller and a pressure roller and subjected to heat and pressure, thereby thermally fusing and fixing the development image onto the sheet of recording paper. The surface temperature of the heating roller must be even to uniformly fix the development image onto the sheet of recording paper and it is necessary for substantially the entire heating roller to be heated uniformly.
However, the power consumption of a heater for heating in the fixing apparatus takes up a large proportion of the overall power consumption of the image forming apparatus, and therefore it is necessary to achieve reductions in the power consumption of the heater for heating in the fixing apparatus in order to reduce the power consumption of the image forming apparatus.
For example, in a heating roller of a fixing apparatus of Patent Document 1 (JP H08-262920A), a plurality of heaters are arranged in a line along a lengthwise direction of the heating roller, and the temperature is raised in only the heaters for heating required to heat surface portions of the heating roller that contact the sheet of recording paper for each sheet of recording paper of various widths, which avoids increasing the amount of heat produced by unneeded heaters, thereby reducing power consumption. Also, a plurality of temperature sensors for detecting the surface temperature of the heating roller in a plurality of locations are provided in order to control the heat produced by the plurality of heaters.
The temperature sensors are thermistors for example and, as disclosed in Patent Document 1 and Patent Document 2 (JP H05-188824A), it is common for the temperature sensors to be made to contact the surface of the heating roller to detect the surface temperature of the heating roller.
On the one hand, since only black developer is used in the case of monochrome images, a developer layer of a monochrome image on a sheet of recording paper is as thin as 20 to 30 μm. In contrast to this, since a color image is formed by superimposing a plurality of colors of developer onto a sheet of recording paper in the case of color images, a developer layer of a color image on a sheet of recording paper becomes as thick as 50 to 80 μm.
For this reason, for monochrome images, a nip region is formed between a metal heating roller coated in Teflon (registered trademark) or titanium and a pressure roller covered by a SiO2 rubber for example, and even though a developer layer can be fixed onto a sheet of recording paper with this nip region, a developer layer of a color image cannot be fixed sufficiently with this nip region.
This is because although it is necessary for the heat of the heating roller to be transmitted sufficiently to the developer layer of a color image in order to thermally fuse the develop layer more than when thermally fusing the developer layer of a monochrome image since the developer layer of a color image is thick, the heat of the heating roller cannot be transmitted sufficiently to the developer layer with the above-described nip region.
Accordingly, in a fixing apparatus for a color image, the surfaces of both the heating roller and the pressure roller are covered with a SiO2 rubber, which thereby achieves an increase in the width of the nip region. With a nip region whose width has been increased, the surface area of the sheet of recording paper that contacts the heating roller is increased, and therefore the heat of the heating roller is sufficiently transmitted to the developer layer on the sheet of recording paper and fixing can be achieved sufficiently by reliably carrying out thermal fusing of the developer layer.
However, by having the temperature sensors come in contact with the surface of the heating roller to control the surface temperature of the heating roller in this color image forming apparatus, damage is caused to the surface of the heating roller by contact with the temperature sensors since the surface of the heating roller is a SiO2 rubber, thus causing unevenness in the fixing.
In particular, since there has been a call for increased printing speed in color image forming apparatuses in recent years and the rotational velocity of the heating roller and the pressure roller in fixing apparatus has been increased, there has been a tendency for more damage to occur on the surface of the heating roller due to contact with the temperature sensors and this has become a cause of reduced image quality.
For this reason, non-contact type temperature sensors have been employed that are capable of detecting the temperature of the heating roller surface without making contact with the surface. However, these non-contact type temperature sensors are large compared with contact-type temperature sensors.
Increasing the size of the heating roller temperature sensors is not desirable due to the advancement of miniaturization of color image forming apparatuses. Providing such large non-contact type temperature sensors in a plurality of locations to detect the surface temperature of the heating roller in the respective locations as described above would require a large space inside the fixing apparatus, which would increase the size of the fixing apparatus, and therefore in turn lead to an increase in size of the color image forming apparatus.